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NUCLEAR CHEMISTRY 200 NISHITH Multimedia India (Pvt.) Ltd., JEE ADVANED - VOL - VI NISHITH Multimedia India (Pvt.) Ltd., SYNOPSIS Nuclear chemistry deals with the changes that occur in the nucleus of the atom. In nuclear chemistry, because the changes in the nucleus are only discussed, the word nuclide is used instead of the word atom. TYPES OF NUCLIDES : ISOTOPES: Isotopes are the atoms of the same element having different mass numbers (A) but same atomic number (Z). Eg: (1)Isotopes of Hydrogen 1 2 3 1 1 1 H , H , H (2)Isotopes of Oxygen 16 17 18 8 8 8 O , O , O Isotopes of the same element differ in the physical properties and radio active properties. . The chemical reactivity of a heavier isotope is always less than that of a lighter isotope, for any element. The atomic weight of an element is a fractional value, this is due to the presence of isotopes in it Eg: Neon has three isotopes 20 21 Ne Ne , and 22 Ne The atomic weight of Neon is 20.183 ISOBARS: Isobars are the atoms of different elements with same mass number but different atomic numbers. Eg: (1) 40 20Ca , 40 40 19 18 K Ar , (2) 14 14 6 7 C N , Isobars have different physical, chemical and radio active properties. ISOTONES: Isotones are the atoms of different elements with different atomic numbers and different mass numbers but same number of neutrons. Eg: (1) 30 31 32 14 15 16 Si P S , , (2) 39 40 19 20 K Ca , Isotones have different physical, chemical and radio active properties. The difference between the number of neutrons and protons in any atom is known as the isotopic number of the atom. Isotopic number = (N - P) = (A - 2 Z) Isotopic number is also known as Neutron excess. ISODIAPHORS: Isodiaphors are the atoms of different elements having different atomic numbers, different mass numbers and different numbers of neutrons, but same isotopic number(A - 2 Z) Eg: (1) 19 23 9 11 F Na , (2) 55 65 24 29 Cr Cu , RADIOACTIVITY Radioactivity may be defined as a process in which nuclei of certain elements spontaneously disintegrate into stable elements. All the heavy elements from bismuth (atomic number 83) through uranium and also a few of the lighter elements possess radioactive properties. However, the radioactive property of the different radioactive elements differs widely, e.g. radium atoms have about three million times the activity of uranium atoms. Uranium in the form of potassium uranyl sulphate, KUO2 (SO4 ) 2 was the first compound found to be radioactive. As radioactive changes are spontaneous, these are not controlled by temperature, pressure or nature of chemical combination. Law of Radioactive Disintegration It is observed that for radioactive decay: i) Atoms of all radioactive elements undergo spontaneous disintegration and form new radioactive elements. The disintegration is accompanied by the emission of α , β , or γ - rays. ii) The disintegration is random, i.e. every atom has equal chance for disintegration at any time. iii) The number of atoms that disintegrate per second is directly proportional to the number of NUCLEAR CHEMISTRY
NISHITH Multimedia India (Pvt.) Ltd., 201 JEE MAINS - CW - VOL - I JEE ADVANCED - VOL - VI NUCLEAR CHEMISTRY NISHITH Multimedia India (Pvt.) Ltd., remaining unchanged radioactive atoms present at any time. The disintegration is independent of all physical and chemical conditions like temperature, pressure chemical combination etc. The natural radioactivity was discovered by Henry Becquerel. In the periodic table all elements after Bismuth (Z = 83) are radio active. Natural radioactive elements are 84 85 86 87 88 89 90 91 92 Po At Rn Fr Ra Ac Th Pa U , , , , , , , , The spontaneous, natural disintegration of unstable atomic nuclei is known as Natural Radio Activity. The spontaneous conversion of one atom into another atom by emitting an particle or particle (or) positron is known as radioactive disintegration or radioactive decay. In natural radio active decay only rays, rays and rays are emitted. rays are streams of particles. -rays are streams of particles. a) particle is the nucleus of helium atom b) particle is a combination of two protons and two neutrons. c) particle is carries 4 units of mass and 2 units of positive charge. d) particle is represented as 4 2 He or He a) particle is electron coming out of the nucleus with very high velocity. b) particle carries neglisible mass and one unit of negative charge c) particle is represented as 0 1 e or a) rays are electro-magnetic radiation having very high frequency (or very low wavelength) b) rays do not contain any particles. c) The wavelength of rays is 14 10 nm to 0.1 nm. a) rays and rays were discovered by Rutherford. b) rays were discovered by Villard. a) rays travel with nearly 1 10 th the velocity of light. b) rays travel with nearly 33% to 99% of the velocity of light. c) rays travel with the velocity of light a) Hard rays have very high velocity b) Soft rays have low velocity In natural radio active decay particle emission (or) particle emission is followed by emission of rays. Emission of particle (or) particle is called primary emission. Emission of particle (or) particle is called primary emission. Velocity order: rays < rays < rays Kinetic energy order rays > rays > rays Penetrating power order: rays < rays < rays (1: 100 : 10,000) Ionising power order: rays > rays > rays (10,000 : 100 : 1) Hard rays have more penetrating power than soft rays. Radio active decay phenomenon depends on the nature of radio active atoms present in the substance. Radio active decay phenomenon will not depend on factors like pressure, temperature, catalyst etc., A radio active element or its compound emits same particles. Eg: 1) 215 84 P emits particle. So a compound having 215 84 0P atoms also emits particles. 2) 211 83Bi emits particle. So a compound having 211 83Bi atoms also emits particles. One mole of particles gives one mole of helium atoms. The volume of 1 mole of helium under STP conditions is 22.4 litres. Only heavy atoms can emit particles. Group displacement law: The result of α and β particle emmission can be summed up in the form of group displacement law. In an α-particle emmission the daughter element has an atomic
NUCLEAR CHEMISTRY 202 NISHITH Multimedia India (Pvt.) Ltd., JEE ADVANED - VOL - VI NISHITH Multimedia India (Pvt.) Ltd., weight less by four units and atomic number less by two units and it falls in a group two columns left to the original element of the periodic table. In a β -particle emmission the daughter element has same atomic weight but its atomic number is increased by one unit than its parent element and hence it lies one column right to the original element of the periodic table. 226 88 Ra belongs to II A group. It emits particle and forms 222 86 Rn which belongs to zero group. 215 84 0P belongs to VI A group. It emits particles and forms 211 82 Pb which belongs to IV A group. 211 83Bi belongs to V A group. It emits particle and forms 211 84 0P which belongs to VI A group. 211 82Pb belongs to V A group. It emits particle and forms 211 83Bi which belongs to V A group. Note: 1) All the elements with atomic numbers 57 to 71 and 89 to 103 belong to III B group. 2) Element of VII A group emits particle and forms element of Zero group. 3) Element of Zero group emits particle and forms element of I A group. 4) Element of I A group emits particle and forms element of VII A group. 238 92U emits particle and forms 234 90Th which belongs to III B group. 234 90Th emits particle and forms 234 91Pa which belongs to III B group. When a radio active nuclide emits one and two particles, its isotope is formed . Eg: 226 222 222 222 88 86 87 88 Ra Rn Fr Ra NUCLEAR STABILITY: The transformation of neutron to proton and vice- versa is a first order reaction. The rate of transformation process depends on relative number of neutrons and protons in the nucleus. In stable nuclei the two changes will be in equilibrium. The fundamental particle unstable outside the nucleus is neutron. The half life period of neutron is 12 minutes. Out side the nucleus the neutron decays giving proton, electron and nutrino. neutron proton + electron + anti nutrino Nutrino and antinutrino have no charge, almost no mass but they have opposite spins. MAGIC NUMBERS: The nuclides having magic number of protons or neutrons are stable. The nuclides having both protons and neutrons in magic numbers are more stable and more abundant. 2,8,20,28,50,82,126 are called magic numbers. Examples for nuclides having both protons and neutrons in magic numbers are 16 4 40 208 8 2 20 82 O He Ca Pb , , , NEUTRON - PROTON RATIO: The stability of the nucleus mainly depends on the N/P value in the nuclide Light nuclide means Z < 20 Heavy nuclide means A > 200 A heavy radio active nuclide exhibits natural radio activity if N P > 1.56 A light nuclide is stable if N P = 1 A light nuclide emits particle if N P > 1 Light nuclides in which N P is less than 1 emit positron 2) undergoes K-capture Heavy nuclides having low N P value emit particles. Eg: 212 208 4 84 0 82 2 P Pb He Nuclides having high N P value are said to be above the zone of stability.
NISHITH Multimedia India (Pvt.) Ltd., 203 JEE MAINS - CW - VOL - I JEE ADVANCED - VOL - VI NUCLEAR CHEMISTRY NISHITH Multimedia India (Pvt.) Ltd., Nuclides having low N P value are said to be below the zone of stability. Nuclides above the zone of stability emit particle. Eg: 1) 14 0 14 6 1 7 C e N 2) 27 0 27 12 1 13 Mg e Al 3) 24 0 24 11 1 12 Na e Mg 4) 3 0 3 1 1 2 H e He Nuclides below the zone of stability generally emit positron. Some nuclides emit particle. Some nuclides capture additional neutron. Eg: 1) 13 0 13 7 1 6 N e C 2) 11 0 11 6 1 5 C e B A light nuclide emits a positron if it contains odd number of protons and odd number of neutrons through the N P value in it is equal to one Eg: 1) 30 0 30 15 1 14 P e Si 2) 22 0 22 11 1 10 Na e Ne Examples for particle emitters 14 35 24 3 10 40 27 C Cl Na H Be K Mg , , , , , , Examples for positron emitters 11 13 27 30 22 38 10 17 C N Si P Na K C F , , , , , , , The region above the stability zone is called decay zone. Nuclides in decay zone emit particles. The region below the stability zone is called positron zone. Nuclides in positron decay zone emit positron. The N P value decreases only in the emission of particle. The N P value increases in 1) emission of particle 2) emission of positron 3) K-electron capture 4) capture of addition neutron The lightest radio active nuclide is Tritium 3 1H The naturally occuring heaviest radio active nuclide is 238 92U Determination of The Number of α -and β - Particles Emitted in a Nuclear Reaction: Consider the following general reaction. ' ' Parent nuclide Daughter nuclide A A Z Z X Y No. of particles ' 4 A A No. of particles Z Z ' 2 no.of particles Law of radioactive decay: According to the law of radioactive decay, the number of atoms of a radio active element which disappears in unit time (rate of disintegration) is directly proportional to the number of atoms of that nuclide present at that moment, hence. t t t t dN dN N or N dt dt Rate of decay of nuclide is independent of temperature, so its energy of activation is zero. Since the rate of decay is directly proportional to the number of atoms of the radioactive nuclide present and as the number of undecomposed atoms decreases with increase in time, the rate of decay also decreases with the increase in time. Various Forms of Equation for Radioactive Decay t N N e t 0 0 t log N log N 2.303 t 2.303 t N N log t 0 t 0 N N log t 2.303 As the above equations are similar to that of first order reaction, hence we can say that radioactive